Compound of 定め(sadame, “fate; rule, law”, the 連用形(ren'yōkei, “stem form”) of verb 定めるsadameru “to decide, to establish, to settle”) +‎ 無い(nai, “there isn't, does not exist”): literally, “there is no set fate”, and by extension, “changeable”.[2]

Note that this is distinct from the negative form of verb 定める (​sadameru), 定めない (​sadamenai), although both have the same reading and inflection.[2]